This invention relates to an instrument for measuring leakage currents in operational amplifiers. More specifically, it relates to a circuit that simultaneously senses the leakage currents at both input terminals of an electrometer operational amplifier while maintaining the amplifier under test in a linear closed-loop configuration.
Electrometer operational amplifiers are, by definition, capable of responding to electrical signals from sources having very high internal resistances, e.g. 10.sup.12 or more ohms. These amplifiers must therefore have correspondingly high input resistances. Equally important, the leakage currents generated by them at their input terminals must be very small, e.g. 10.sup.-12 or less amperes. Production testing of the amplifiers requires measurement of the leakage currents for each amplifier. In this context, repeatable measurements require a resolution of 10.sup.-14 amperes. These tests are difficult and time-consuming because of the small currents involved. Specifically, with such small currents it takes as much as several seconds for the necessary filter capacitances and stray capacitances in the measuring circuitry to charge up so that accurate readings can be taken. In addition, error currents due to stray leakage paths can degrade the accuracy of the test, making the design and layout of the input signal path crucial.
It is a principal object of the invention to provide a circuit for sensing the leakage currents of an electrometer amplifier in less time than was heretofore feasible. Further, this invention eliminates the need for relays in the signal path which are a prime source of stray leakage error.
A more specific object of the invention is to provide a circuit that is capable of relatively quick measurement of the leakage currents of an electrometer amplifier and yet is characterized by accuracy and repeatability of measurement.
Other objects of the invention will be in part obvious and will in part appear hereinafter.